Comparing the low-res screen grabs upscaled to 1920x1080 in Photoshop to the screen grabs of the 6GB capture, the low-res screen grabs have tons of ringing and MPEG artifacts that are practically non-existent on the HD screen grabs, there's more fine detail in the HD versions, and the colors are richer and more vibrant in the HD versions. I'm also not seeing these scanlines you're talking about, unless you're referring to the lines around the woman on the electronic billboard, which are in the source material and just a special effect to simulate monitor scanlines.

I've seen what high-end scalers can do--I've owned several, including an HTPC that put all of the black box scalers to shame. None of them can perform the type of processing it would take to clean up those SD screen grabs and make them look like the HD versions. Faroudja used to be regarded as king of the hill, but they've been resting on their laurels and their scalers are usually regarded as overpriced and underfeatured compared to the competition these days. The Algolith Mosquito is the only processor on the market right now with really good MPEG noise reduction capabilities, but it's not a scaler. HTPCs can perform noise reduction and scaling, but again, they can't make those SD screen grabs look like the HD screen grabs.

And while the HD screen grabs do look a tad soft, we don't know when this master was created, who performed the telecine conversion, or the condition of the original film stock.

I'd say it's very unlikely the HD version is an upconversion unless it's an upconversion from an entirely different and newer SD master than the DVD--and studios don't do SD masters these days.